1. Field of the Disclosure
This disclosure relates to the field of electronics and, in particular, to the application of devices that can be used to locate objects that are concealed or situated underground.
2. Related Art
Line locators are used to locate the position of lines buried in the ground, such as gas pipes, water pipes, telephone cables, and power cables. A line locator typically includes a transmitter and a receiver. The transmitter causes an electromagnetic field to be radiated from the line to be located. The receiver locates the line by detecting the radiated field. Line locators may be operated in one of two modes, a direct connection mode and an inductive mode.
In direct connection mode, the transmitter is physically connected to the line to be located, typically at a point in the line that is above the ground. The transmitter generates a voltage at one end of the line, causing an electrical current to flow along the conductive line. The current produces an electromagnetic field around the line, which can be detected by the receiver of a line locator.
In cases where direct connection to the line is not possible, for example if the line is completely underground, line locators typically operate in an inductive mode. In inductive mode the line locator produces current in the line by electromagnetic induction: A time-varying electromagnetic field is radiated by an antenna, which induces an electrical current in the line to be located. The current in the line then produces an electromagnetic field around the line and the electromagnetic field is detected by a receiver in the line locator.
To locate the line, an operator typically moves the receiver over the surface of the ground until the presence of an electromagnetic field is indicated and, therefore, the location of the line is determined. The receiver measures the strength of the electromagnetic field, which is proportional to the current in the buried, or target, line. According to Ohm's Law, the current in the line is inversely proportional to the impedance of the line. Because of the wide variability of the physical condition of buried lines and the wide variability of the environment of these lines, the impedance of buried lines can have virtually any value. However, in most situations a particular current can be induced so that the line can generate an electromagnetic field.
Line locators available today are effective in locating and tracing buried lines. However, there are situations in which the use of a line locator can result in the improper determination of the location of the line, which can result in damage to the line or serious injury to a person excavating near the line. Such a situation can occur when there are structures or objects in the vicinity of a target line that affect the measurements made by the receiver. An example of this effect is when there are several lines buried in the ground in close proximity to the target line. The electrical current in the target line can induce electrical currents in the adjacent lines, whereby the electromagnetic field generated by the target line is distorted. A distorted field can result in an incorrect determination of the location of the line as well as an inaccurate measurement of depth. Also, the locator may find an adjacent line other than the target line due to bleed-over of the electromagnetic field from the target line onto the adjacent line.
One method that can be utilized to deal with the effects of distorted fields is to measure and record the strength of the electromagnetic field and the spatial position at which each measurement of the electromagnetic field strength is taken. For such a system to be useful in many situations, the system would be required to have a positional accuracy of less than about one inch over a range of several hundred feet, and the system would need to operate in real-time with measurements made at a rate on the order of ten times per second.
One way to determine the position of each measurement of field strength is to use the Global Positioning System (GPS). In this approach, a GPS receiver is operated together with the line locating receiver and the data from both is recorded. The problem with the use of GPS for this purpose is that the accuracy of GPS or even Differential GPS (DGPS), is not generally sufficient. The expected accuracy of GPS is from ten to twenty feet, whereas the expected accuracy of DGPS is from three to five feet. This accuracy is not sufficient for dealing with distorted electromagnetic fields from underground lines. Although some embodiments of GPS provide an accuracy of less than one inch, these measurements cannot be made in real time as the receiver of the locator is moved at reasonable rates spatially over the area of interest.
Another problem in the field of underground line location is the need to keep track of the position of buried lines over periods that last more than a few hours or days. Presently, operators who perform underground line location often mark the location of the line by painting lines on the ground above the buried line. However, it is often necessary to locate the lines with respect to landmarks in the vicinity of the lines in the event that the paint wears away or is moved. At present line locations are maintained by using a tape measure or other external measuring devices to create lines between the landmarks or remaining painted lines. However, as stated above, the accuracy required for line location often needs to be less than one inch. Tape measures used in this fashion do not provide the required level of accuracy. Further, the length of time required to obtain accurate positional data for line locator measurements using a tape measure is generally prohibitive.
Therefore, there is a need for a line locating system capable of measuring the strength of an electromagnetic field and simultaneously and accurately measuring the spatial position associated with the measurement of field strength. There is also a need for a line locating system that can accurately maintain a determination of the location of underground lines for long periods of time so that those lines can be determined at a later date.